Large-scale layer 2 metropolitan area network

ABSTRACT

A system and method permits the creation of very-large metropolitan area networks (MANs) using Layer  2  (L 2 ) switching technology. Different groups of L 2  switches are logically organized into Islands. Connected to each Island are a plurality of customers sites, and an interconnect fabric couples the Islands together. The Islands cooperate to provide a Virtual Ethernet Connection (VEC) to each set of customer sites being coupled together. Customers identify their traffic that corresponds to a VEC by labeling or tagging it with a Customer-Equipment VLAN Identifier (CE-VLAN ID). Within each Island, the CE-VLAN ID specified by the customer&#39;s traffic (and hence the corresponding VEC) is mapped to a unique MAN Provider-Equipment VLAN ID (PE-VLAN ID). To prevent the formation of loops, the Islands run the Inter-MAN Control Protocol (IMCP), which represents a modified version of the Multiple Spanning Tree Protocol (MSTP).

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to computer networks and, morespecifically, to large-scale metropolitan area networks.

[0003] 2. Background Information

[0004] Many organizations, including businesses, governments andeducational institutions, utilize computer networks so that employeesand others may share and exchange information and/or resources. Acomputer network typically comprises a plurality of entitiesinterconnected by means of one or more communications media. An entitymay consist of any device, such as a computer, that “sources” (i.e.,transmits) or “sinks” (i.e., receives) data frames over thecommunications media. A common type of computer network is a local areanetwork (“LAN”) which typically refers to a privately owned networkwithin a single building or campus. LANs typically employ a datacommunication protocol (LAN standard), such as Ethernet, FDDI or tokenring, that defines the functions performed by data link and physicallayers of a communications architecture (i.e., a protocol stack).

[0005] One or more intermediate network devices are often used to coupleLANs together and allow the corresponding entities to exchangeinformation. For example, a bridge may be used to provide a “switching”function between two or more LANs or end stations. Typically, the bridgeis a computer and includes a plurality of ports that are coupled viaLANs either to other bridges, or to end stations such as routers or hostcomputers. Ports used to couple bridges to each other are generallyreferred to as a trunk ports, whereas ports used to couple bridges toend stations are generally referred to as access ports. The bridgingfunction includes receiving data from a sending entity at a source portand transferring that data to at least one destination port forforwarding to one or more receiving entities.

Ethernet

[0006] Ethernet is one of the most common LAN standards used today. Theoriginal Ethernet transmission standard, referred to as 10 Base-T, iscapable of transmitting data at 10 Megabits per second (Mbs). In 1995,the Institute of Electrical and Electronics Engineers (IEEE) approved aFast Ethernet transmission standard, referred to as 100 Base-T, which iscapable of operating at 100 Mbs. Both 10 Base-T and 100 Base-T, however,are limited to cable lengths that are less than 100 meters. A committeeof the IEEE, known as the 802.3z committee, is currently working onGigabit Ethernet, also referred to as 1000 Base-X (fiber channel) and1000 Base-T (long haul copper), for transmitting data at 1000 Mbs. Inaddition to the substantially increased transmission rate, GigabitEthernet also supports cable lengths of up to 3000 meters. GigabitEthernet thus represents a potentially significant increase in the sizeor range of Ethernet LANS.

Spanning Tree Algorithm

[0007] Most computer networks include redundant communications paths sothat a failure of any given link does not isolate any portion of thenetwork. Such networks are typically referred to as meshed or partiallymeshed networks. The existence of redundant links, however, may causethe formation of circuitous paths or “loops” within the network. Loopsare highly undesirable because data frames may traverse the loopsindefinitely.

[0008] Furthermore, some devices, such as bridges or switches, replicateframes whose destination is not known resulting in a proliferation ofdata frames along loops. The resulting traffic can overwhelm thenetwork. Other intermediate devices, such as routers, that operate athigher layers within the protocol stack, such as the Internetwork Layerof the Transmission Control Protocol/Internet Protocol (“TCP/IP”)reference model, deliver data frames and learn the addresses of entitieson the network differently than most bridges or switches, such thatrouters are generally not susceptible to sustained looping problems.

[0009] To avoid the formation of loops, most bridges and switchesexecute a spanning tree protocol which allows them to calculate anactive network topology that is loop-free (i.e., a tree) and yetconnects every pair of LANs within the network (i.e., the tree isspanning). The IEEE has promulgated a standard (IEEE Std. 802.1D-1998™)that defines a spanning tree protocol to be executed by 802.1Dcompatible devices. In general, by executing the 802.1D spanning treeprotocol, bridges elect a single bridge within the bridged network to bethe “Root Bridge”. The 802.1D standard takes advantage of the fact thateach bridge has a unique numerical identifier (bridge ID) by specifyingthat the Root Bridge is the bridge with the lowest bridge ID. Inaddition, for each LAN coupled to any bridge, exactly one port (the“Designated Port”) on one bridge (the “Designated Bridge”) is elected.The Designated Bridge is typically the one closest to the Root Bridge.All ports on the Root Bridge are Designated Ports, and the Root Bridgeis the Designated Bridge on all the LANs to which it has ports.

[0010] Each non-Root Bridge also selects one port from among itsnon-Designated Ports (its “Root Port”) which gives the lowest cost pathto the Root Bridge. The Root Ports and Designated Ports are selected forinclusion in the active topology and are placed in a forwarding state sothat data frames may be forwarded to and from these ports and thus ontothe LANs interconnecting the bridges and end stations of the network.Ports not included within the active topology are placed in a blockingstate. When a port is in the blocking state, data frames will not beforwarded to or received from the port. A network administrator may alsoexclude a port from the spanning tree by placing it in a disabled state.

[0011] To obtain the information necessary to run the spanning treeprotocol, bridges exchange special messages called configuration bridgeprotocol data unit (BPDU) messages or simply BPDUs. BPDUs carryinformation, such as assumed root and lowest root path cost, used incomputing the active topology. More specifically, upon start-up, eachbridge initially assumes itself to be the Root Bridge and transmitsBPDUs accordingly. Upon receipt of a BPDU from a neighboring device, itscontents are examined and compared with similar information (e.g.,assumed root and lowest root path cost) stored by the receiving bridgein memory. If the information from the received BPDU is “better” thanthe stored information, the bridge adopts the better information anduses it in the BPDUs that it sends (adding the cost associated with thereceiving port to the root path cost) from its ports, other than theport on which the “better” information was received. Although BPDUs arenot forwarded by bridges, the identifier of the Root Bridge iseventually propagated to and adopted by all bridges as described above,allowing them to select their Root Port and any Designated Port(s).

[0012] In order to adapt the active topology to changes and failures,the Root Bridge periodically (e.g., every hello time) transmits BPDUs.In response to receiving BPDUs on their Root Ports, bridges transmittheir own BPDUs from their Designated Ports, if any. Thus, BPDUs areperiodically propagated throughout the bridged network, confirming theactive topology. As BPDU information is updated and/or timed-out and theactive topology is re-calculated, ports may transition from the blockingstate to the forwarding state and vice versa. That is, as a result ofnew BPDU information, a previously blocked port may learn that it shouldbe in the forwarding state (e.g., it is now the Root Port or aDesignated Port).

Rapid Spanning Tree Protocol

[0013] Recently, the IEEE promulgated a new standard (the IEEE Std.802.1W-2001™ specification standard) that defines a Rapid Spanning TreeProtocol (RSTP). The RSTP similarly selects one bridge of a bridgednetwork to be the Root Bridge and defines an active topology thatprovides complete connectivity among the LANs while severing any loops.Each individual port of each bridge is assigned a port role according towhether the port is to be part of the active topology. The port rolesdefined by the 802.1w specification standard include Root, Designated,Alternate and Backup. The bridge port offering the best, e.g., lowestcost, path to the Root Port is assigned the Root Port Role. Each bridgeport offering an alternative, e.g., higher cost, path to the Root Bridgeis assigned the Alternate Port Role. For each LAN, the one portproviding the lowest cost path to the Root Bridge from that LAN isassigned the Designated Port Role, while all other ports coupled to theLAN are assigned the Root, Backup or, in some cases, the Alternate PortRole. At the Root Bridge, all ports are assigned the Designated PortRole.

[0014] Those ports that have been assigned the Root Port and DesignatedPort Roles are placed in the forwarding state, while ports assigned theAlternate and Backup Roles are placed in a state. A port assigned theRoot Port Role can be rapidly transitioned to the forwarding stateprovided that all of the ports assigned the Alternate Port Role areplaced in the blocking state. Similarly, if a failure occurs on the portcurrently assigned the Root Port Role, a port assigned the AlternatePort Role can be reassigned to the Root Port Role and rapidlytransitioned to the forwarding state, provided that the previous RootPort has been transitioned to the discarding or blocking state. A portassigned the Designated Port Role or a Backup Port that is to bereassigned to the Designated Port Role can be rapidly transitioned tothe forwarding state, provided that the roles of the ports of thedownstream bridge are consistent with this port being assigned theDesignated Port Role. The RSTP provides an explicit handshake to be usedby neighboring bridges to confirm that a new Designated Port can rapidlytransition to the forwarding state.

[0015] Like the STP described in the 802.1D specification standard,bridges running RSTP also exchange BPDUs in order to determine whichroles to assign to the bridge's ports. The BPDUs are also utilized inthe handshake employed to rapidly transition Designated Ports to theforwarding state.

Virtual Local Area Networks

[0016] A computer network may also be segmented into a series of logicalnetworks. For example, U.S. Pat. No. 5,394,402, issued Feb. 28, 1995 toRoss (the “'402 Patent”), discloses an arrangement for associating anyport of a switch with any particular network segment. Specifically,according to the '402 Patent, any number of physical ports of aparticular switch may be associated with any number of groups within theswitch by using a virtual local area network (VLAN) arrangement thatvirtually associates the port with a particular VLAN designation. Morespecifically, the switch or hub associates VLAN designations with itsports and further associates those VLAN designations with messagestransmitted from any of the ports to which the VLAN designation has beenassigned.

[0017] The VLAN designation for each port is stored in a memory portionof the switch such that every time a message is received on a givenaccess port the VLAN designation for that port is associated with themessage. Association is accomplished by a flow processing element whichlooks up the VLAN designation in the memory portion based on theparticular access port at which the message was received. In many cases,it may be desirable to interconnect a plurality of these switches inorder to extend the VLAN associations of ports in the network. Thoseentities having the same VLAN designation function as if they are allpart of the same LAN. VLAN-configured bridges are specificallyconfigured to prevent message exchanges between parts of the networkhaving different VLAN designations in order to preserve the boundariesof each VLAN. Nonetheless, intermediate network devices operating aboveL2, such as routers, can relay messages between different VLAN segments.

[0018] In addition to the '402 Patent, the IEEE promulgated the 802.1Qspecification standard for Virtual Bridged Local Area Networks. Topreserve VLAN associations of messages transported across trunks orlinks in VLAN-aware networks, both Ross and the IEEE Std. 802.1Q-1998specification standard disclose appending a VLAN identifier (VID) fieldto the corresponding frames. In addition, U.S. Pat. No. 5,742,604 toEdsall et al. (the “'604 patent”), which is commonly owned with thepresent application, discloses an Interswitch Link (ISL) encapsulationmechanism for efficiently transporting packets or frames, includingVLAN-modified frames, between switches while maintaining the VLANassociation of the frames. In particular, an ISL link, which may utilizethe Fast Ethernet standard, connects ISL interface circuitry disposed ateach switch. The transmitting ISL circuitry encapsulates the frame beingtransported within an ISL header and ISL error detection information,while the ISL receiving circuitry strips off this information andrecovers the original frame.

Multiple Spanning Tree Protocol

[0019] The IEEE is also working on a specification standard for aSpanning Tree Protocol that is specifically designed for use withnetworks that support VLANs. The Multiple Spanning Tree Protocol (MSTP),which is described in the IEEE 802.1s draft specification standard,organizes a bridged network into regions. Within each region, MSTPestablishes an Internal Spanning Tree (IST) which provides connectivityto all bridges within the respective region and to the ISTs establishedwithin other regions. The IST established within each MSTP Region alsoprovides connectivity to the one Common Spanning Tree (CST) establishedoutside of the MSTP regions by IEEE Std. 802.1Q-1998 compatible bridgesrunning STP or RSTP. The IST of a given MST Region receives and sendsBPDUs to the CST. Accordingly, all bridges of the bridged network areconnected by a single Common and Internal Spanning Tree (CIST). From thepoint of view of the legacy or IEEE 802.1Q bridges, moreover, each MSTRegion appears as a single virtual bridge on the CST.

[0020] Within each MST Region, the MSTP compatible bridges establish aplurality of active topologies, each of which is called a MultipleSpanning Tree Instance (MSTI). The MSTP bridges also assign or map eachVLAN to one and only one of the MSTIs. Because VLANs may be assigned todifferent MSTIs, frames associated with different VLANs can takedifferent paths through an MSTP Region. The bridges may but typically donot compute a separate topology for every single VLAN, therebyconserving processor and memory resources. Each MSTI is basically asimple RSTP instance that exists only inside the respective Region, andthe MSTIs do not interact outside of the Region.

[0021] MSTP, like the other spanning tree protocols, uses BPDUs toestablish the ISTs and MSTIs as well as to define the boundaries of thedifferent MSTP Regions. The bridges do not send separate BPDUs for eachMSTI. Instead, every MSTP BPDU carries the information needed to computethe active topology for all of the MSTIs defined with the respectiveRegion. Each MSTI, moreover, has a corresponding Identifier (ID) and theMSTI IDs are encoded into the bridge IDs. That is, each bridge has aunique ID, as described above, and this ID is made up of a fixed portionand a settable portion. With MSTP, the settable portion of a bridge's IDis further organized to include a system ID extension. The system IDextension corresponds to the MSTI ID. The MSTP compatible bridges withina given Region will thus have a different bridge ID for each MSTI. For agiven MSTI, the bridge having the lowest bridge ID for that instance iselected the root. Thus, an MSTP compatible bridge may be the root forone MSTI but not another within a given MSTP Region.

[0022] Each bridge running MSTP also has a single MST ConfigurationIdentifier (ID) that consists of three attributes: an alphanumericconfiguration name, a revision level and a VLAN mapping table thatassociates each of the potential 4096 VLANs to a corresponding MSTI.Each bridge, moreover loads its MST Configuration ID into the BPDUssourced by the bridge. Because bridges only need to know whether or notthey are in the same MST Region, they do not propagate the actual VLANto MSTI tables in their BPDUs. Instead, the MST BPDUs carry only adigest of the VLAN to MSTI table or mappings. The digest is generated byapplying the well-know MD-5 algorithm to the VLAN to MSTI table. When abridge receives an MST BPDU, it extracts the MST Configuration IDcontained therein, including the digest, and compares it to its own MSTConfiguration ID to determine whether it is in the same MST Region asthe bridge that sent the MST BPDU. If the two MST Configuration IDs arethe same, then the two bridges are in the same MST Region. If, however,the two MST Configuration IDs have at least one non-matching attribute,i.e., either different configuration names, different revision levelsand/or different computed digests, then the bridge that received theBPDU concludes that it is in a different MST Region than the bridge thatsourced the BPDU. A port of an MST bridge, moreover, is considered to beat the boundary of an MST Region if the Designated Bridge is in adifferent MST Region or if the port receives legacy BPDUs.

[0023]FIG. 1 is a highly schematic block diagram of an MST BPDU 100. TheMST BPDU 100 includes a header 102 compatible with the Media AccessControl (MAC) layer of the respective LAN standard, e.g., Ethernet. Theheader 102 comprises a destination address (DA) field, a source address(SA) field, a Destination Service Access Point (DSAP) field, and aSource Service Access Point (SSAP), among others. The DA field 104carries a unique bridge multicast destination address assigned to thespanning tree protocol, and the DSAP and SSAP fields carry standardizedidentifiers assigned to the spanning tree protocol. Appended to header102 is a BPDU message area that includes an “outer” part 104 and an“inner” part 106. The outer part 104 has the same format as an RSTP BPDUmessage and is recognized as a valid RSTP BPDU message by bridges thatdo not implement MSTP. The “inner” part 106 is utilized by bridgesexecuting MSTP to establish the IST and the MSTIs. The inner part 106has a set of spanning tree parameters for the IST and a set ofparameters for each MSTI supported by the bridge sourcing the MSTP BPDU100.

[0024] Outer part 104, also referred to as the CIST priority vector, hasa plurality of fields, including a protocol identifier (ID) field 108, aprotocol version ID field 110, a BPDU type field 112, a flags field 114,a CIST root ID field 116, an external path cost field 118, a CISTregional root ID field 120, a CIST port ID field 122, a message agefield 124, a maximum (MAX) age field 126, a hello time field 128, and aforward delay field 130. The CIST root identifier field 116 contains theidentifier of the bridge assumed to be the root of the Common andInternal Spanning Tree, which may be in the same MSTP Region as thebridge sourcing the BPDU message 100, in another MSTP Region or in partof the bridged network that is not running MSTP. The external path costfield 118 contains a value representing the lowest cost from the bridgesourcing the BPDU 100 to the CIST root identified in field 116 withoutpassing through any other bridge in the same region as the bridge thatis sourcing the BPDU message 100.

[0025] Inner part 106, also referred to as an MSTI priority vector,similarly has a plurality of fields, including a version 1 length field132, a null field 134, a version 3 length field 136, an MSTconfiguration ID field 138, a CIST regional root ID field 140, a CISTregional path cost field 142, a CIST bridge ID field 144, a CIST port IDfield 146, a CIST flags field 148, and a CIST hops field 150. Inner part106 may further include one or more optional MSTI configuration messages152, each of which constitutes another MSTI priority vector or M-record.

[0026] Because version 2 of the RSTP does not specify any additionalfields beyond those already specified by version 1, the MST BPDU doesnot have a version 2 length field.

[0027] As mentioned above, the MST configuration ID field 138 is made upof three sub-fields: a configuration name sub-field 154, a revisionlevel sub-field 156 and an MD-5 checksum sub-field 158. Theconfiguration name sub-field 154 carries a variable length text stringencoded within a fixed size, e.g., 32-octets. The revision levelsub-field 156 carries an integer encoded within a fixed field of twooctets. The MD-5 checksum sub-field 158 carries a 16-octet signaturecreated by applying the MD-5 algorithm to the bridge's VLAN to MSTItable, which contains 4096 consecutive two octet elements.

[0028] Each MSTI Configuration Message 152 consists of a plurality offields including a CIST regional root ID field 160, a CIST regional pathcost field 162, a CIST bridge ID field 164, a CIST port ID field 166, aCIST flags field 168 and a CIST hops field 170. MST bridges utilize theSTP parameters contained in fields 140-150 of inner part 106 and in eachMSTI configuration message 152 to compute an active topology for eachMSTI configured in the respective region.

Metropolitan Area Networks (MANs)

[0029] Multiple LANs and/or end stations may be interconnected bypoint-to-point links, microwave transceivers, satellite hook-ups, etc.to form a metropolitan area network (MAN) that typically spans severalcity blocks, an entire city and/or an entire metropolitan area, such asthe San Francisco Bay Area. The MAN typically interconnects multipleLANs and/or end stations located at individual campuses and/or buildingsthat are physically remote from each other, but that are still withinthe metropolitan area. Conventional MANs typically rely on networkequipment employing Asynchronous Transfer Mode (ATM) running over theexisting Public Switched Telephone Network's (PSTN's) SynchronousOptical Network (SONET). As most LANs utilize the Ethernet standard,network messages or packets created at one LAN must be converted fromEthernet format into ATM cells for transmission over the SONET links.The ATM cells must then be converted back into Ethernet format fordelivery to the destination LAN or end station. The need to convert eachnetwork message from Ethernet to ATM and back again requires the MAN toinclude expensive networking equipment. The MAN Provider also has tolease or otherwise obtain access to the SONET links. As a result, MANscan be expensive to build and operate.

[0030] Accordingly, a need exists for a system and method for buildingand operating MANs more efficiently.

SUMMARY OF THE INVENTION

[0031] Briefly, the invention is directed to a system and method forbuilding very-large metropolitan area networks (MANS) using Layer 2 (L2)switching technology. In the illustrative embodiment, different groupsof L2 switches are logically organized into Islands. Each Island,moreover, is configured as a separate administrative domain. Connectedto each Island are a plurality of customers sites, which are typicallylocal area networks (LANs). An interconnect fabric is utilized to couplethe Islands together so that a customer site connected to a first Islandcan communicate with a customer site connected either to the same or asecond Island. In the illustrative embodiment, the interconnect fabricis formed from a plurality of Layer 3 (L3) devices configured to providean Emulated VLAN over Multiple Label Switching Protocol (EVoMPLS)service, where EVoMPLS is the analogy, over MPLS, of ATM LAN Emulation(ATM Forum standard af-lane-0021.000). Alternatively, the interconnectfabric may be formed of an Ethernet LAN using 802.1Q or similar tags.The Islands cooperate to provide a Virtual Ethernet Connection (VEC) toeach set of customer sites being coupled together. Customers identifytheir traffic that corresponds to a VEC by labeling or tagging it with aCustomer-Equipment VLAN Identifier (CE-VLAN ID). Within each Island, theCE-VLAN ID specified by the customer's traffic (and hence thecorresponding VEC) is mapped to a unique MAN Provider-Equipment VLAN ID(PE-VLAN ID). The PE-VLAN ID selected for a given VEC in one Island maydiffer from the PE-VLAN ID selected for the given VEC but used inanother Island. For each VEC that traverses the interconnect fabric, anInter-Island Trunk is established to carry VEC traffic between the twoIslands. The Inter-Island Trunk is a logical construct that functions,at least from the point of view of the Islands, as a shared medium.Specifically, the Islands joined by an Inter-Island Trunk are configuredto append the same Virtual Circuit Identifier (ID), preferably as anMPLS label, to network messages being placed on the Inter-Island Trunk.Network messages received at an Island from the Inter-Island have theirVirtual Circuit ID label and any other labels stripped off before beingtransmitted to the respective customer site.

[0032] The concatenated MAN consisting of Islands and interconnectfabric may be expected to be too large for any of the standard SpanningTree Protocols to serve satisfactorily to prevent the formation ofloops. To prevent the formation of loops within the MAN, the Islands areconfigured to prevent two or more VECs from sharing the sameInter-Island Trunk. The Islands also run a new protocol, the Inter-MANControl Protocol (IMCP), which represents a modified version of theMultiple Spanning Tree Protocol (MSTP). Specifically, the L2 devicesdisposed in each Island are configured with a new Multiple Spanning Tree(MST) Configuration ID that includes an Island name in addition to theconfiguration name, revision level and checksum. Furthermore, the L2devices disposed in the same Island are all given the same Island ID,configuration name and revision level. Each Island thus identifiesitself as a separate MSTP Region. Second, the L2 devices within eachIsland also ensure that, for each VEC that crosses the interconnectfabric, all but one of the redundant links connecting the Island to theinterconnect fabric are blocked. As a result, loops that might otherwiseresult from the presence of redundant links between the customer sitesand the Islands are severed, regardless of the version of the STP beingrun in the customer sites. For different VECs, however, the links thatare blocked may vary, thereby providing a level of load-sharing betweenthe links extending between the Islands and the interconnect fabric.

[0033] The IMCP also imposes several new rules. In particular, BPDUsreceived within an Island whose entire MST Configuration ID matches thatof the receiving L2 device are treated as normal, matching BPDUs.Received BPDUs whose Island name matches the Island name of thereceiving L2 device, but whose configuration name, revision level and/orconfiguration digest does not match are treated as Rapid Spanning TreeProtocol (RSTP) BPDUs. This rule allows bridges in the same Island tooperate in the same manner as for 802.1S, and maintain connectivityduring the reconfiguration of the bridges. Received BPDUs whose Islandname does not match the Island name of the L2 device receiving the BPDUsand whose specified Root ID does not match that of the receiving L2device are ignored, if received from the Inter-Island Trunk. This ruleeffectively decouples the Islands' Spanning Trees from each other. Ifreceived on a bridge port other than an Inter-Island Trunk, the receiptof a BPDU whose Island name does not match the Island name of the L2device receiving the BPDU causes the receiving bridge to block therespective port for all VLANs and issue an operator alarm. This ruleprevents inadvertent connections among Islands other than on anInter-Island Trunk. In the preferred embodiment, the L2 devices alsorespond to receiving BPDUs whose Island name does not match the Islandname of the L2 device receiving the BPDUs but whose specified Root IDdoes match that of the receiving L2 device by blocking the respectiveport for all VLANs and issue an operator alarm. This rule allows anIsland to detect inadvertent connections among Islands which are nototherwise detected.

BRIEF DESCRIPTION OF THE DRAWINGS

[0034] The invention description below refers to the accompanyingdrawings, of which:

[0035]FIG. 1, previously discussed, is a schematic block diagram of aconventional configuration bridge protocol data unit in accordance withthe Multiple Spanning Tree Protocol;

[0036]FIG. 2 is a highly schematic illustration of a large MetropolitanArea Network (MAN);

[0037]FIG. 3 is a highly schematic illustration of an Island of the MANof FIG. 2;

[0038]FIG. 4 is a partial, functional diagram of a Layer 2 (L2) deviceof the Island of FIG. 3;

[0039]FIG. 5 is a highly schematic illustration of a ConfigurationIdentifier (ID);

[0040]FIG. 6 is a highly schematic illustration of a VLAN Mapping Table;

[0041]FIG. 7 is a highly schematic illustration of an Inter-Island TrunkMapping Table;

[0042]FIG. 8 is a highly schematic illustration of a labeled networkmessage format;

[0043] FIGS. 9-11 are highly schematic partial illustrations of the MANof FIG. 2; and

[0044]FIGS. 12 and 13 are highly schematic illustrations of another MANin accordance with the present invention.

DETAILED DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT

[0045]FIG. 2 is a highly schematic illustration of a very large layer 2(L2) Metropolitan Area Network (MAN) 200 in accordance with the presentinvention. As used herein, the term “very large MAN” refers to a MANcapable of covering an entire metropolitan area, such as the SanFrancisco Bay area, Silicon Valley, etc. The MAN 200 includes aplurality of Islands, such as Islands 202, 204 and 206. As describedherein, each Island comprises one or more interconnected Layer 2 (L2)intermediate network devices, such as bridges or switches. Typically,each Island is operated by the same MAN Provider, and represents aseparate administrative domain. The MAN 200 is organized into differentIslands to increase the total number of VLAN designations beyond 4096that may be supported by the MAN 200. Some or all of the individualIslands, moreover, may be assigned to different administrators.

[0046] The Islands are coupled together by an Island Interconnect Fabric208. Preferably, each Island is coupled to the Island InterconnectFabric 208 by multiple links, such as Inter-Island links 210 a-f. Also,attached to each Island are one or more customer sites, such as personalcustomers, sites 212-217. In the illustrative embodiment, each customersite comprises a plurality of entities or hosts, such as personalcomputers, workstations, servers, etc., which are all in the samephysical location, and are interconnected to form one or more Local AreaNetworks (LANs) so that the entities may source or sink data frames toone another. As used herein, the term “same physical location” refers toa single building or a plurality of buildings on a single campus orwithin the area of roughly a single city block. The LANs at the customersites may be interconnected by one or more customer operated L2intermediate network devices such as bridges, switches or routers.

[0047] Customer sites 212-217 will typically belong to differentorganizations, such as organization A and organization B. In particular,organization A includes customer sites 212 (A1), 213 (A2), 216 (A3) and217 (A4). Organization B includes customer sites 214 (B1), and 215 (B2).Each customer site 212-217 is preferably coupled to at least one Islandby a plurality of site links 220-231. As described herein, a customerobtains various services from the MAN 200, such as interconnecting itssites that are geographically remote from each other. In this way,entities located at one customer site can communicate with the entitiesof another site.

[0048] The MAN 200 of FIG. 2 is meant for illustration purposes only andis not meant to limit the invention. Indeed, MAN 200 will typicallyinclude many more customer sites, e.g., thousands.

[0049]FIG. 3 is a highly schematic illustration of Island 202. Island202 includes a plurality of L2 intermediate network devices, such asswitches (S) 302, 304 and 306. Each switch 302, 304 and 306 includes aplurality of ports (P) 402 at least some of which are utilized toconnect the switches to the customer sites. Other switch ports 402 arecoupled to intra-Island links 308-310 extending between the switches302, 304 and 306. Links 308-310 may be point-to-point links or sharedmedia links that carry network messages, such as frames, among theswitches. Each switch 302-306, moreover, preferably identifies its ownports 402, e.g., by port numbers, such as port zero (P0), port one (P1),port two (P2), port three (P3), etc. Switches 302-306 are thus able toassociate specific ports with the customer sites and/or other switchescoupled thereto.

[0050] At least some of the switches of each Island may be classifiedinto different categories. For example, each Island has one or moreProvider Edge switches, which are disposed at the boundary between theIsland and one or more customer sites. The Provider Edge switches aredirectly coupled to the customer sites. Each Island also includes one ormore Island Boundary Bridges that connect the Island to the IslandInterconnect Fabric 208. With reference to FIG. 3, switch 304 is aProvider Edge Bridge, switch 306 is an Island Boundary Bridge and switch302 is both a Provider Edge Bridge and an Island Boundary Bridge.

[0051] Suitable intermediate network device platforms for use with thepresent invention include, but are not limited to, the commerciallyavailable Catalyst 4000 and 6000 series of switches from Cisco Systems,Inc. of San Jose, Calif.

[0052]FIG. 4 is a partial block diagram of MAN Provider switch, such asswitch 302. Switch 302 includes a plurality of ports 402 a-402 e each ofwhich is preferably identified by a number (e.g., P0-P4). One or moreframe transmission and reception objects, designated generally 404, areassociated with the ports 402 a-d such that network messages, includingframes, received at a given port, e.g., P3, may be captured, and framesto be transmitted by switch 302 may be delivered to the appropriateport, e.g., P1, for transmission. Frame reception and transmissionobjects 404 are preferably message storage structures, such as priorityqueues. In the illustrated embodiment, switch 302 includes transmittingand receiving circuitry, including one or more line cards and/or networkinterface cards (NICs) establishing ports for the exchange of networkmessages, one or more supervisor cards having central processing units(CPUs) and/or microprocessors and associated memory devices forperforming computations and storing the results therefrom and one ormore bus structures.

[0053] Switch 302 has a plurality of protocol entities, including atleast one Multiple Spanning Tree Protocol (MSTP) entity 408, at leastone forwarding engine 410, and a Virtual Ethernet Channel (VEC) entity412. The MSTP entity 408 preferably comprises a plurality ofsubcomponents, including a port role selection state machine 414, a porttransition state machine 416, a bridge protocol data unit (BPDU) messagegenerator 418, an Island Boundary Determination engine 420, and an MSTPDigest Generator 422. Island Boundary Determination engine 420preferably includes one or more comparators, such as comparator 423. TheMSTP entity 408 preferably operates in accordance with the IEEE 802.1sMultiple Spanning Tree Protocol (MSTP) draft supplement to the 802.1Qspecification standard, the current draft (IEEE Draft P802.1s/D13™—Jun.13, 2002) of which is hereby incorporated by reference in its entirety,as modified by the Inter-MAN Control Protocol (IMCP) described herein.The MSTP entity 408 includes or is in communicating relationship with amemory device or structure, such as STP memory 424, which may be avolatile or non-volatile random access memory (RAM) or some other memorydevice. Memory 424 is preferably organized to include a plurality ofrecords or cells (not shown) for storing spanning tree relatedinformation or parameters such as the switch's Configuration ID, numericbridge identifier (ID), the assigned path cost for each port 402 a-e foreach MSTI, the current or “best” spanning tree information for each portP0-P4 for each MSTI, etc. In addition to memory 424, the STP entity 408further includes a VLAN ID (VID) to Multiple Spanning Tree Instance(MSTI) translation table 426 configured to store the mappings of VLANsto MSTIs.

[0054] The VEC entity 412 comprises a VLAN mapping engine 428, a tagmanipulation engine 430, and an Inter-Island Trunk engine 432. The VLANmapping engine 428 includes one or more VLAN mapping tables 600 thatmaps Customer Equipment VLAN IDs (CE-VLANs) to Provider Equipment VLANIDs (PE-VLANs) preferably on a per port basis. In an alternativeembodiment, there may be a separate VLAN mapping engine and a separateVLAN mapping table for each port (or some number of ports), and eachVLAN mapping table may be configured with a different mapping ofCE-VLANs to PE-VLANs.

[0055] The Inter-Island Trunk Mapping engine 432 has an Inter-IslandTrunk Mapping table 700 that maps PE-VLAN IDs to VEC Identifiers (IDs).VEC IDs are preferably appended to frames prior to transmission into theIsland Interconnect Fabric 208. To provide connectivity betweendifferent customer sites, VEC entity 412 is configured to establish oneor more User Network Interface (UNIs), such as UNI 01 also designated byreference numeral 436 and UNI 02 also designated by reference numeral438. As described herein, each UNI represents the termination point ofone or more VECs, and may thus be considered to define one or morelogical VEC ports. UNI 436, for example, has three VEC ports 440 a-c.UNI 438 has two VEC ports 442 a-b.

[0056] The forwarding engine 410 is in communicating relationship withthe frame transmission and reception objects 404 and is coupled to atleast one filtering database 444 that stores address informationcorresponding to the entities of the MAN 200 (FIG. 2). Specifically,filtering database 444 has a plurality of records (not shown) eachcontaining a plurality of cells, including a destination address cell, adestination port cell, a filtering database ID (FID) cell and acorresponding timer cell. Each record in the filtering database 444preferably corresponds to a particular network entity. The FID, which isderived from the message's PE-VLAN ID, allows a given destination MACaddress to correspond to the same or to different MAC addresses fordifferent PE-VLAN IDs. The forwarding engine 410 is configured to switchor bridge network messages, such as packets and/or frames, from a sourceport 402 to one or more destinations ports 402 depending on informationcontained in the forwarding database 428 and also on the spanning treeport states of the respective ports 402 as managed by MSTP entity 408.The forwarding engine 410 is also in communicating relationship with theMSTP entity 408 and relays MSTP-related messages received at ports 402thereto. Forwarding engine 410 may also be in communicating relationshipwith VEC entity 412.

[0057] It will be understood by those skilled in the art that MSTPentity 408, forwarding engine 410 and VEC entity 412 may each compriseregisters and combinational logic configured and arranged to producesequential logic circuits. In the illustrated embodiment, MSTP entity408, forwarding engine 410 and VEC entity 412 are preferably acombination of software modules or libraries containing programinstructions pertaining to the methods described herein, which areexecutable by one or more processing elements (not shown) of switch 302,and hardware elements. Other computer readable media may also be used tostore and execute these program instructions. Nonetheless, those skilledin the art will recognize that various combinations of software andhardware, including firmware, may be utilized to implement the presentinvention.

Formation of Islands as Separate Administrative Domains

[0058] Initially, the MAN Provider organizes its equipment, i.e., L2switches 302-306, into a plurality of Islands. In the preferredembodiment, the switches are organized into Islands by configuring theirMST Configuration IDs in a specific manner. More specifically, the MANProvider first decides which of its switches should be organized into agiven Island. The MAN Provider then configures the MST Configuration IDfor every switch within the given Island to be the same. FIG. 5 is ahighly schematic illustration of an MST Configuration ID 500 configuredin accordance with the present invention. The MST Configuration ID 500has a 32-byte Island Name field 502, a 32-byte Configuration Name field503, a 2-byte Revision Level field 504 and a 2-byte Configuration Digestfield 506, which is preferably formed by applying the MD-5 ChecksumAlgorithm to the contents of the VID/MSTI Translation Table 426 (FIG.4). For each switch in the given Island, the MAN Provider configures theIsland Name field 502, the Configuration Name field 503 and the RevisionLevel field 504 of each switch's MST Configuration ID 500 with the samevalues. The MAN Provider also establishes the same mappings of PE-VLANsto MSTIs with the given Island. Thus, each switch in the given Islandwill generate the same digest value. The switches store the MSTConfiguration IDs selected by the MAN Provider at their STP memories424.

[0059] In the preferred embodiment, the Island Name field 502 is 2-bytesor longer, the Configuration Name field 503 is 32-bytes, the RevisionLevel field 504 is 2-bytes and the Configuration Digest field 506 is2-bytes. In an alternative embodiment, the Island Name field 502 andConfiguration Name field 503 are a combined 32-bytes and the two valuesmay be separated by some specially selected character, such as the “#”symbol.

[0060] The configuration of switches 302-306 may be performed locallythrough a Command Line Interface (CLI) provided at the switch orremotely through the well-known Simple Network Management Protocol(SNMP).

[0061] For example, switches 302, 304 and 306 (FIG. 3), which are alldisposed in Island 202, are each be configured with the same Islandname, e.g., “ISLAND0001”, the same configuration name, e.g., “MAN4452”and the same revision level, e.g., “0001”. Switches 302, 304 and 306will additionally be configured to have the same mapping of PE-VLANs toMSTP Instance IDs. The switches disposed in Island 204 (FIG. 2), on theother hand, will each be configured with a different Island name, e.g.,“ISLAND0002”. They may each be configured with the same or a differentconfiguration name and/or revision level and will typically beconfigured with a different mapping of PE-VLANs to MSTP Instance IDs.

[0062] When the MAN Provider initializes and runs its switches, theywill automatically, i.e., without manual intervention, segregatethemselves into the desired Islands as part of their execution of theMSTP. More specifically, because switches 302, 304 and 306 have eachbeen configured with the same Island names, the same configurationnames, the same revision level numbers and the same mapping of PE-VLANsto MSTP Instance IDs, they will conclude that they are all part of thesame MSTP Region or, in this case, the same Island.

[0063] As described herein, the UNIs are configured to treat BPDUmessages received from a customer site in one of two ways. Specifically,the UNIs either discard BPDU messages received from the customer site ortreat the received BPDU messages as data frames and tunnel them throughthe Island so that they may be received by other customer sites.Similarly, the UNIs do not send BPDUs generated by the Provider EdgeBridges into the customer sites. Accordingly, the MAN Provider'sswitches 302, 304 and 306 do not cooperate in the calculation of anyactive topology(ies) with the intermediate network devices located inthe customer sites.

[0064] Within each Island, moreover, the MAN Provider's switches willestablish an active topology for each MSTP Instance defined within therespective Island. Suppose, for example, that the MAN Provider definedten MSTP Instances within Island 202 and assigned at least one PE-VLANto each MSTP Instance. Switches 302, 304 and 306, as part of theirexecution of the MSTP, will establish an Internal Spanning Tree (IST) aswell as ten loop-free, active topologies within Island 202.

Linking Multiple Customer Sites Through One or More Islands

[0065] Suppose the customer operating sites 212, 213, 216 and 217 (FIG.2) wishes to interconnect these sites. More specifically, suppose thatthe customer wishes to connect site 212 with site 213, site 212 withsite 216 and site 216 with site 217. The customer preferably contactsthe MAN Provider and requests such services. The MAN Provider, in turn,configures its equipment, i.e., the switches disposed in Islands 202,204 and 206 to establish the desired connections.

[0066] In accordance with the present invention, the MAN Providerprovides the requested service by establishing a Virtual EthernetConnection (VEC) between each of the identified customer sites. A VECsimulates a physical Ethernet link or an Ethernet bridged LAN extendingbetween two or more customer sites. As described herein, within eachIsland, there is a one-to-one correspondence between a VEC and a PE-VLANID.

[0067] As shown by the network illustrated in FIG. 2, customer sites 212and 213 are both connected to the same Island, i.e., to Island 202.Customer sites 212 and 216, however, are each connected to differentIslands, i.e., to Islands 202 and 204, respectively. Similarly, customersites 216 and 217 are each connected to a different Island, i.e., toIslands 204 and 206, respectively. To provide the requestedconnectivity, the MAN Provider, among other things, preferablyestablishes a first VEC that connects customer sites 212 and 213, asecond VEC that connects customers sites 212 and 216, and a third VECthat connects sites 216 and 217. The first VEC resides entirely withinIsland 202. Accordingly, the MAN Provider simply needs to establish anIntra-Island Link 240 within Island 202 for use by the first VEC.

[0068] It should be understood that an Intra-Island Link is simply alogical representation of an interconnection between two customer sitesacross a single Island that, in the preferred embodiment, is a VLANoperating in accordance with the IEEE Std. 802.1Q-1998 specificationstandard. The Intra-Island Link may additionally or alternatively employthe ISL protocol from Cisco Systems, Inc.

[0069] The second VEC, on the other hand, must span multiple Islands,i.e., Islands 202 and 204. Accordingly, the MAN Provider must establishan Inter-Island Trunk 242 that connects Islands 202 and 204 for use bythe second VEC. The third VEC similarly spans multiple Islands and thusit too requires access to an Inter-Island Trunk 244 that couples Islands204 and 206.

Intra-Island Links

[0070] Creation of the first VEC which couples customer sites 212 and213 preferably proceeds as follows. Within Provider Edge switch 302,which connects to customer site 212, the MAN Provider establishes a UserNetwork Interface (UNI), such as UNI 438 (FIG. 4). A UNI is a logicalinterface between a customer site and the MAN Provider's network, e.g.,an Island. Each UNI established by the MAN Provider has one or more VECports each of which represents a termination or end point of acorresponding VEC that has been created by the MAN Provider. Within UNI438, VEC port 442 a may be assigned to the first VEC. The MAN Providerthen assigns a MAN Provider Equipment VLAN ID (PE-VLAN ID) to the firstVEC. As described herein, the PE-VLAN ID is a VLAN designation that isappended to and thus identifies frames travelling through the respectiveIsland, e.g., Island 202, that correspond to a respective VEC, e.g., thefirst VEC which connects customers sites 212 and 213. The PE-VLAN ID forthe first VEC may be “4011”.

[0071] The customer selects a Customer Equipment VLAN ID (CE-VLAN ID) tobe used by the customer when communicating between customer sites 212and 213 coupled by Island 202. The CE-VLAN ID, which may be “0014”, istypically selected based on the needs of the customer's own sites andits networking equipment. The customer configures its own equipment sothat all network messages, e.g., Ethernet frames, created in one of thesites, e.g., site 212, that are to be delivered to the other site, e.g.,site 213, are tagged with the chosen CE-VLAN ID. The MAN Provider learnsof the selection and configures the VEC entities 412 of the switchesthat are at the Island's boundaries and that connect to the two customersites, i.e., switches 302 and 304 of Island 202, to map the chosenCE-VLAN ID, i.e., “0014”, to the respective PE-VLAN ID, i.e., “4011”. Inparticular, the MAN Provider configures the Customer VLAN mapping table600 of the VEC entities 412.

[0072]FIG. 6 is a highly schematic illustration of VLAN mapping table600 of switch 302. Table 600 is organized at least logically as a tableor array having a plurality of columns and rows whose intersectionsdefine cells or records for storing information. Table 600 preferablyhas a CE-VLAN ID column 602, a VEC column 604, a PE-VLAN ID column 606,a UNI column 608, and a VEC Port column 610. Table 600 also has aplurality of rows 614 a-c. The MAN Provider preferably assigns a freerow, e.g., row 614 a, to the first VEC. At row 614 a, the MAN Providerloads the chosen CE-VLAN ID, i.e., “0014”, into the cell correspondingto column 602, a VEC ID, e.g., “001”, into the cell corresponding tocolumn 604, the PE-VLAN ID, e.g., “4011”, that has been assigned to thechosen CE-VLAN ID into the cell corresponding to column 606, theparticular UNI assigned to this VEC, i.e., UNI 01, into the cellcorresponding to column 608, and the particular VEC Port, i.e., VEC Port0, into the cell corresponding to column 610. The MAN Provider similarlyconfigures the VLAN mapping table 600 of switch 304 which is at theboundary of Island 202 and customer site 213.

[0073] End stations in the two sites 212 and 213 can now communicatewith each other by using the chosen CE-VLAN ID. Suppose, for example,that a workstation disposed in site 212 wishes to communicate with aworkstation in site 213. The workstation in site 212 encodes its messageinto one or more Ethernet frames, and in the frames' VLAN ID fieldinserts the CE-VLAN ID chosen by the customer, i.e., “0014”. These VLANID tagged frames are received by switch 302 within Island 202, which isat the boundary to customer site 212. The VLAN ID tagged frames areinitially provided to the switch's VEC entity 412, which accesses itsVLAN mapping table 600 to perform a look-up. Specifically, the VECentity 412 searches table 600 to determine to which VEC the receivedframes belong. Row 614 a of the VLAN mapping table 600 indicates thatCE-VLAN ID “0014” corresponds to VEC “001” and that this VEC has beenmapped to PE-VLAN ID “4011”.

[0074] In one embodiment of the present invention, the VEC entity's tagmanipulation engine 430 loads the frames' VLAN ID fields with PE-VLAN ID“4011”, replacing CE-VLAN ID “0014”. Alternatively, the tag manipulationengine 430 may add a new VLAN Identifier (VID) field (not shown) to themessage and load this new VID field with the respective PE-VLAN ID,i.e., “4011”, leaving the original VID field (carrying the CE-VLAN ID)unmodified.

[0075] The frames, which are now tagged with PE-VLAN ID “4011”, are thenprovided to the UNI for transmission via the VEC Port that has beenestablished for this VEC. The frames travel on the Intra-Island Link 240established for the VEC and are received at switch 304. As indicatedabove, the Intra-Island Link 240 basically corresponds to a portion ofthe MSTP Instance or active topology defined within Island 202 to whichPE-VLAN ID “4011” has been mapped. To the extent the frames areforwarded by any intermediary switches or bridges disposed in-betweenswitches 302 and 304, these intermediary switches preferably do notmodify the frames. That is, the frames do not undergo any furtherchanges to their VLAN tags by switches that are forwarding the frames toother switches within Island 202.

[0076] At switch 304, the frames are received on a VEC Port thatrepresents the other end of the VEC created to interconnect customersites 212 and 213. As the frames are about to be transmitted from theUNI at switch 304, i.e., they are about to be transmitted outside ofIsland 202, they are subjected to another transformation. Morespecifically, the frames are provided to the VEC entity 412 of switch304, which performs a look-up on its VLAN mapping table 600. Here, VECentity 412 searches table 600 based on the PE-VLAN ID with which theframes have been tagged. The VEC entity 412 determines that PE-VLAN ID“4011” corresponds to CE-VLAN ID “0014”. Accordingly, the tagmanipulation engine 430 loads the frames' VLAN ID fields with CE-VLAN ID“0014”, replacing PE-VLAN ID “4011”. The frames, which have beenrestored with their original VLAN IDs, are then sent from switch 302into customer site 213. The frames are then delivered to the targetedworkstation based on the destination address carried by the frames.

[0077] In the embodiment where the new VID field is added to the frameupon receipt in the Island 202, the tag manipulation engine 430 atswitch 304 strips off the new VID field before sending the frame intocustomer site 213.

[0078] Communication from the workstation in customer site 212 to site213 works in a similar manner. Specifically, at switch 304, the frameswhich are tagged with the assigned CE-VLAN ID are modified by loadingthe PE-VLAN ID that is assigned to this CE-VLAN ID into the frames' VLANID field. The frames then travel along the Intra-Island Link 240 withinIsland 202 to switch 302 which is at the boundary with customer site212. The frames are then restored with their original CE-VLAN IDs andtransmitted into customer site 212 for delivery to the targetedworkstation of customer site 212.

[0079] It should be understood that different CE-VLAN IDs could havebeen selected within customer sites 212 and 213 for use with the firstVEC. In this case, the VLAN Mapping table 600 is preferably configuredto specify both CE-VLAN IDs.

Inter-Island Trunks

[0080] Creation of the second VEC coupling customer sites 212 and 216preferably proceeds as follows. Within switch 302, which connects tocustomer site 212, the MAN Provider either establishes a new UNI orassigns an existing UNI to the second VEC. As UNI 438 is alreadyassigned to customer site 212 for purposes of the first VEC, the MANProvider may re-use this existing UNI 438 for the second VEC.Nonetheless, a new VEC Port at UNI 438, such as VEC Port 442 b, must beprovided for the second VEC as each VEC must have its own VEC port. TheMAN Provider then selects and assigns a PE-VLAN ID to the second VEC foruse within Island 202. The selected PE-VLAN ID will be used to identifyframes travelling through the Island 202 that correspond to the secondVEC. Suppose that the MAN Provider selects PE-VLAN ID “4027” for thesecond VEC within Island 202.

[0081] A CE-VLAN ID is chosen by the customer for use by networkentities disposed in customer site 212 when communicating with networkentities disposed in customer site 216. Suppose the customer choosesCE-VLAN ID “0038” for use in customer site 212. The customer configuresits own networking equipment disposed within site 212 so that allnetwork messages, e.g., Ethernet frames, created within that site anddestined for network entities in site 216 are tagged with CE-VLAN ID“0038”. The customer also notifies the MAN Provider of the selectedCE-VLAN ID. In response, the MAN Provider then configures the VEC entity412 of switch 302 which is at the boundary between Island 202 andcustomer site 212 to map frames tagged with the chosen CE-VLAN ID, i.e.,“0038”, to the selected PE-VLAN ID, i.e., “4027” that is being mappedthereto. In particular, the MAN Provider configures the VLAN mappingtable 600 of the VEC entity 412 at switch 302.

[0082] More specifically, the MAN Provider assigns a free row, e.g., row614 b, to the second VEC. At row 614 b, the MAN Provider loads thechosen CE-VLAN ID, i.e., “0038”, into the cell corresponding to column602, a VEC ID, e.g., “002”, into the cell corresponding to column 604,and the corresponding PE-VLAN ID, e.g., “4027”, selected by the MANProvider into the cell corresponding to column 606. The MAN Provideralso loads the particular UNI assigned to this VEC, i.e., UNI 01, intothe cell corresponding to column 608, and the selected VEC Port, i.e.,VEC Port 1, into the cell corresponding to column 610.

[0083] Within Island 204, which connects to customer site 216, the MANProvider establishes a UNI that gives network entities in customer site216 access to the second VEC. The UNI is preferably provided at theProvider Edge switch(es) at the boundary between Island 204 and site216, i.e., the switch(es) that are directly connected to customer site216, i.e., via site links 228 and/or 229. The MAN Provider alsoestablishes a VEC port within the UNI to terminate the second VEC atIsland 204. The MAN Provider then selects and assigns a PE-VLAN ID tothe second VEC for use within Island 204. The selected PE-VLAN ID willbe used to identify frames travelling within Island 204 that correspondto the second VEC. Notably, the selected PE-VLAN ID for use in Island204 may be different from PE-VLAN ID “4027” which was selected for usein Island 202. Indeed, suppose that the MAN Provider selects PE-VLAN ID“4017” for the second VEC within Island 204.

[0084] As above, the customer chooses a CE-VLAN ID based on its ownneeds and the capabilities of its networking equipment to be used bynetwork entities disposed in customer site 216 when communicating withnetwork entities disposed in customer site 212. The CE-VLAN ID that ischosen for use in site 216 may be the same or may differ from the oneselected for use in customer site 212. Suppose the customer selectsCE-VLAN ID “0018” for use in customer site 216. The customer configuresits own internetworking equipment disposed within site 216 so that allnetwork messages, e.g., Ethernet frames, created within that site anddestined for network entities in site 212 are tagged with CE-VLAN ID“0018”. The customer also notifies the MAN Provider of the selectedCE-VLAN ID. The MAN Provider then configures the VEC entity 412 of theswitch disposed in Island 204 that is directly connected to customersite 216 to map frames tagged with CE-VLAN ID “0018” to the PE-VLAN IDselected for use in Island 204, i.e., “4017”. In particular, the MANProvider configures the VLAN mapping table 600 of the VEC entity 412 atthe Provider Edge switch(es) of Island 204 relative to customer site216.

[0085] Row 614 c (FIG. 6) illustrates how the VLAN Mapping Table 600 atthe respective Provider Edge switch(es) of Island 204 are configured.More specifically, the MAN Provider loads the chosen CE-VLAN ID, i.e.,“0018”, into the cell corresponding to column 602, the VEC ID, e.g.,“002”, into the cell corresponding to column 604, and the correspondingPE-VLAN ID, e.g., “4017”, that has been mapped to the chosen CE-VLAN IDinto the cell corresponding to column 606. The MAN Provider also loadsthe UNI assigned to this VEC, e.g., UNI 00, into the cell correspondingto column 608, and the VEC Port, e.g., VEC Port 0, into the cellcorresponding to column 610.

[0086] As shown, this second VEC passes through two separate Islands 202and 204 in order to provide connectivity between the two selectedcustomer sites 212 and 216. Accordingly, the second VEC utilizes anInter-Island Trunk 242, which extends through the Island InterconnectFabric 208 and connects the two Islands 202 and 204. In the illustrativeembodiment, the Inter-Island Trunks operate as shared-medium Ethernet orbridged LAN in connectivity, and are established by Multiple ProtocolLabel Switching (MPLS) virtual private networks (VPNs), Packet Ring,Asynchronous Transfer Mode (ATM) Emulated LAN, or other suchtechnologies. The MPLS VPNs are formed within the Island InterconnectFabric 208. Notably, each VEC that crosses the Island InterconnectFabric 208 must only use a single Inter-Island Trunk. Nonetheless,multiple VECs may use the same Inter-Island Trunk.

[0087] In the illustrative embodiment, the Island Interconnect Fabric208 may be the well-known Internet.

[0088] As indicated above, each of the MAN Provider's Islands includesat least one Island Boundary Bridge which is the switch or bridge thatprovides direct connectivity from the Island to the Island InterconnectFabric 208, and thus to the other Islands of the MAN Provider'sMetropolitan Area Network. At Island 202, for example, switches 302 and306 are both Island Boundary Bridges because they provide directconnectivity to the Island Interconnect Fabric 208 via Inter-Islandlinks 210 a and 210 b, respectively. For those VECs, such as the secondVEC, that utilize an Inter-Island Trunk, the MAN Provider must configurethe VEC entities located in the Island Boundary Bridges of the two (ormore) Islands being interconnected to modify the frames for transmissionacross the Inter-Island Trunk. First, the MAN Provider configures theInter-Island Trunk Mapping Table 700 of the Island Boundary Bridges.

[0089]FIG. 7 is a highly schematic illustration of an Inter-Island TrunkMapping Table 700. Table 700 is organized at least logically as a tableor array having a plurality of columns and rows whose intersectionsdefine cells or records for storing information. Table 700 preferablyhas a PE-VLAN ID column 702, a VEC column 704, and an Inter-Island TrunkID column 706. Table 700 also has a plurality of rows 710 a-c. At theIsland Boundary Bridges in Island 202, the MAN Provider preferablyassigns a free row, e.g., row 710 a, to the second VEC. At row 710 a,the MAN Provider loads the selected PE-VLAN ID for Island 202, i.e.,“4027”, into the cell corresponding to column 702, and a VEC ID selectedfor the second VEC, e.g., “002”, into the cell corresponding to column704. The MAN Provider loads the cell corresponding to column 706 with anInter-Island Trunk ID corresponding to the tag or label that is to beappended to network messages traversing the Island Interconnect Fabric208. The Inter-Island Trunk ID, which may comprise more than one labelor tag, is selected depending on the particular protocol(s) used tointerconnect the Islands. Assuming that the MPLS protocol and, morespecifically, Emulated VLAN over MPLS (EVoMPLS) is the protocol beingused, a unique MPLS label, e.g., “6042”, is selected for the secondVEC's Inter-Island Trunk.

[0090] The MAN Provider also configures the Inter-Island Trunk MappingTable 700 at the Island Boundary Bridge(s) of Island 204. Row 710 b(FIG. 7) illustrates how this entry would be configured. Specifically,PE-VLAN ID “4017” which was selected for use in Island 204 is loadedinto the cell corresponding to column 702, the common VEC ID, i.e.,“002”, is loaded into the cell corresponding to column 704, and thecommon Inter-Island Trunk ID is loaded into the cell corresponding tocolumn 706.

[0091] Second, the MAN Provider configures the Island Boundary Bridgesto tag frames for transmission over the Inter-Island Trunk and tocapture and process frames received over the Inter-Island Trunk. Inparticular, when switch 302 of Island 202 receives a frame tagged withCE-VLAN ID “0038” which corresponds to the second VEC, it uses theCE-VLAN ID to perform a look-up on its VLAN Mapping Table 600 to derivethe corresponding PE-VLAN ID, i.e., “4027”. Switch 302 then replaces theCE-VLAN ID with corresponding PE-VLAN ID and forwards the frame intoIsland 202 (assuming the VEC is utilizing Island link 210 b at switch306). The frame is received at switch 306, which encapsulates thereceived frame for transmission across the Island Interconnect Fabric208.

[0092]FIG. 8 is a highly schematic illustration of an encapsulated frame800 for transmission across Island Interconnect Fabric 208. Theencapsulated frame 800 includes an MPLS label stack 802 appended to theoriginal Ethernet frame 804. As indicated above, if the PE-VLAN ID wasadded to the CE-VLAN ID at the UNI, instead of replacing it, then theEthernet frame 804 may include a VLAN ID (VID) field 805, correspondingto the CE-VLAN ID. The MPLS label stack 802 includes a Layer 2 (L2)header 806 that corresponds to the medium employed by the IslandInterconnect Fabric 208, an IP/MPLS header 808 and a Virtual EthernetCircuit ID field 810. A suitable encapsulation scheme for use with thepresent invention is described in Request for Comments (RFC) 2684Multiprotocol Encapsulation over ATM Adaptation Layer 5 (September1999). The Island Boundary Bridge performs a look-up on its Inter-IslandTrunk Mapping Table 700 to derive the Virtual Ethernet Circuit ID.Specifically, the Island Boundary Bridge locates the Inter-Island TrunkID that corresponds to the PE-VLAN ID with which the received Ethernetframe is tagged. Here, the PE-VLAN ID is “4027” and the matchingInter-Island Trunk ID is “6042”. This retrieved value is loaded into theVirtual Ethernet Circuit ID field 810.

[0093] The encapsulated frame is then transmitted onto the IslandInterconnect Fabric 208. The Inter-Island Trunk 242 delivers the frameto all ports within Islands 202-206 that are members of the same VEC asspecified by the Virtual Ethernet Circuit ID (other than the port onwhich the frame was sent). The encapsulated frame is thus received atthe Island Boundary Bridge(es) of Island 204. The Island Boundary Bridgeof Island 204 utilizes the value loaded in the encapsulated frame'sVirtual Ethernet Circuit ID field to derive the corresponding PE-VLAN IDfor use in Island 204. Here, the Virtual Ethernet Circuit ID is “6042”and the matching PE-VLAN ID from row 710 b (FIG. 7) is thus “4017”. TheIsland Boundary Bridge also determines whether it can accept thereceived frame based on the spanning tree state of the port on which itwas received. If the port is in the blocking spanning tree port statefor VLAN “4017”, the frame is discarded. In this case, there would beanother Island Boundary Port at Island 204 that is in the forwardingspanning tree port state for VLAN “4017”, and could thus accept theframe.

[0094] The Island Boundary Bridge of Island 204 at which the frame isaccepted strips off the MPLS label stack and recovers the originalEthernet frame 804. In the frame's VLAN ID field 805, the IslandBoundary Bridge loads the PE-VLAN ID for this VEC, i.e., “4017”. TheIsland Boundary Bridge then transmits the frame within Island 204. Theframe is received at the Provider Edge bridge of Island 204 for customersite 216. The Provider Edge bridge performs a look-up on its VLANMapping Table 600 using the frame's PE-VLAN ID to derive thecorresponding CE-VLAN ID. Here, the PE-VLAN ID is “4017” and thus thematching CE-VLAN ID is “0018”. Accordingly, the Provider Edge bridgeloads the CE-VLAN ID into the Ethernet frame replacing the PE-VLAN ID.The frame, tagged with the CE-VLAN ID, is then transmitted by theProvider Edge switch of Island 204 into customer site 216 for receipt bythe target network entity.

Preventing the Formation of Loops

[0095] As shown in FIG. 2, each customer site is preferably coupled toits respective Island by multiple links. In addition, each Island isconnected to the Island Interconnect Fabric 208 by multiple links. Inorder to take advantage of the fast convergence time of RSTP (as opposedto the 802.1D spanning tree protocol), each Island preferably has atmost two connections or links to any one Inter-Island Trunk. The use ofmultiple links prevents any customer site and/or Island from losingconnectivity should any consistent link fail. The presence of multiplelinks, however, can result in the formation of loops as both thecustomer sites and the Islands are operating at layer 2 (L2) as opposedto some higher layer of the communication stack. Specifically, becausethe Provider Edge Bridges do not cooperate with the customer networks inthe execution of any Spanning Tree Protocol, each UNI transitions to theforwarding spanning tree state for each PE-VLAN ID defined in theIsland. Thus, the UNIs do not discard any frames received from thecustomer networks, unless a CE-VLAN ID maps to no PE-VLAN ID in the VLANMapping Table 600.

[0096] Loops formed by the presence of redundant links between acustomer site and an Island are preferably severed by having thecustomer site block one or more of its ports. This may be achieved bytreating BPDUs generated in a customer site exactly the same as dataframes. More specifically, each UNI at the Provider Edge Bridges may beconfigured to examine the destination MAC address of frames receivedfrom the respective customer site to determine whether the addressmatches a destination MAC address utilized by BPDU messages. If so, theframe is recognized as such by the Provider Edge Bridge. In response,the Provider Edge Bridge preferably transports the BPDU message, like adata frame, through the Provider network. In order to prevent thecustomer generated BPDU from being mistaken by the provider's switchesfor a provider generated BPDU, the customer BPDU may be altered oningress to the provider network, and restored on egress, for example byaltering its destination MAC address. When the BPDU is received back atthe same or another customer site, it is processed in accordance withthe particular Spanning Tree Protocol operating in the customer site ina conventional manner. As a result, the Provider network will appear tothe customer site simply as a shared-medium, and the customer'sinternetworking equipment, through operation of a Spanning Tree Protocol(STP), will sever the loop by blocking either a port facing one of theUNIs or a port that is internal to the customer site.

[0097] Alternatively, the Provider Edge Bridges may be configured tosimply discard BPDUs that are received at the UNI. In this case, a loopmay exist, resulting in the rate of frames entering the Island from thecustomer site continuing to increase. If the Island monitors the rate ofdata being received from the customer site, then a warning may betriggered if this rate exceeds some threshold.

[0098] To avoid the formation of loops resulting from the presence ofmultiple connections between a given Island and the Island InterconnectFabric 208, the Islands preferably run a new protocol, the Inter-MANControl Protocol (IMCP) in accordance with the present invention. TheIMCP, which represents a modified version of MSTP, specifies specialrules and methods to efficiently prevent the formation of loops amongthe Islands of a MAN Provider's Metropolitan Area Network. This modifiedversion blocks the formation of loops and yet avoids having to run asingle instance of the spanning tree protocol across the entire MAN,i.e., across all of the Islands. Indeed, because there may be hundredsof Islands (if not more) and because the total-number of VECs definedwithin the Islands may be much greater than the 4096 permitted by theIEEE Std. 802.1Q-1998 and IEEE Draft P802.1s/D13 specificationstandards, it would be impractical if not impossible to run a spanningtree instance across them.

[0099] As indicated above, the MAN Provider configures the VID/MSTITranslation Table 426 of the switches in each Island so as to associateeach PE-VLAN ID with exactly one MSTI. Within a given Island, theswitches within the given Island will typically support a plurality ofMSTIs and one CIST. Traffic corresponding to different PE-VLAN IDs canthus be load-shared among the different active topologies defined by theMSTIs and the CIST.

[0100] For redundancy (and load-sharing) purposes, each Islandpreferably has at most two connections to each Inter-Island Trunk. Forexample, an Island may have a single Inter-Island Bridge with twoconnections to an Inter-Island Trunk and/or two Inter-Island Bridgeseach having one connection to that Inter-Island Trunk.

[0101] In addition to forwarding network messages to and from the portscoupled to Inter-Island Trunks, also referred to as Inter-Island Ports,Island Boundary Bridges also generate and send BPDUs from their ports,including these ports. In particular, the BPDU message generators 418 ofthe Island Boundary Bridge's MSTP entities 408 formulate MST BPDUmessages 100 having the form shown in FIG. 1. The message generators 418access the MSTP entity's STP memory 424 for the information used ingenerating the BPDUs. In particular, the MSTP entity's digest generator422 produces a digest value from its VID/MSTI Translation Table 426using the MD-5 algorithm. The BPDU message generator 418 then retrievesthe Island name, Configuration ID and Revision Level from the STP memory424, and creates the MST Configuration ID 500, which is preferablyinserted into field 138 (FIG. 1) of the MSTP BPDU 100. The BPDU message418 similarly retrieves STP parameter values from STP memory 424 forloading into the other fields of the BPDU 100.

[0102] In addition to the VEC ID established for each VEC traversing anInter-Island Trunk, an extra VEC and corresponding VEC ID is defined tobe used only by IMCP. In particular, the extra VEC ID is used with BPDUstransmitted by the Island Boundary Bridges into the Inter-Island Trunk.That is, the extra VEC ID is loaded in the MPLS Label Stack 802 appendedto BPDUs prior to transmission into the Island Interconnect Fabric 208.In the illustrative embodiment, every bridge within an Island that runsIMCP and has a port onto a VEC assigned to a given Inter-Island Trunkalso has a port onto the extra VEC. Encapsulated messages received at anIsland Boundary Bridge, such as switch 306, that carry the VEC ID forthe extra VEC are recognized as Island generated BPDUs. In response, theVirtual Ethernet Channel entity 412 strips off the encapsulation,recovers the BPDU and passes the BPDU to the MSTP entity 408 forprocessing.

[0103] Row 710 c may correspond to an entry for the extra VEC, i.e., VEC“301”, used in Inter-Island Trunk 242, i.e., “6042”, as data VEC “002”.As shown, no PE-VLAN ID is assigned to the extra VEC as BPDUs receivedby an Island Boundary Bridge are not forwarded. The assigned VEC ID isloaded into the Virtual Ethernet Circuit ID field 810 of encapsulatedBPDUs prior to transmission into Inter-Island Trunk 242.

[0104] As described above, an Inter-Island Trunk functions like ashared-medium Ethernet or a bridged LAN in connectivity. Thus, BPDUstransmitted onto an Inter-Island Trunk are received by all otherswitches “coupled” to the Inter-Island Trunk as well as by other portsof the switch transmitting the BPDU that also happen to be coupled tothe Inter-Island Trunk. Accordingly, BPDUs issued from one Inter-IslandPort and encapsulated with the extra VEC ID are delivered to allInter-Island Ports (other than the port on which they were sent) coupledto the Inter-Island Trunk. The switches, moreover, utilize theinformation in the received BPDUs to compute an active topology for eachMSTI defined at the switch. As a result, for each PE-VLAN ID, an Islandwill block all but one Island link 210 to the respective Inter-IslandTrunk. Because each VEC is associated with a single PE-VLAN ID withineach Island, moreover, all but one of the Inter-Island links 210 foreach VEC will be blocked. The particular Island link 210 thattransitions to the forwarding state may, moreover, vary among PE-VLANIDs. This provides a measure of load-sharing among the Inter-Islandlinks 210.

[0105] When a MAN Provider switch disposed in an Island, including anInter-Island Bridge, receives a BPDU, it passes the BPDU to the MSTPentity 408. If the BPDU was received on an Inter-Island Port, it willhave been encapsulated within an MPLS label stack. In this case, theBPDU like all such messages are passed to the VEC entity 412. The VECentity 412 determines that the message is encapsulated with the extraVEC ID. In this case, the VEC entity 412 strips off the MPLS label stackand passes the BPDU to the MSTP entity 408. If the BPDU is an MSTP BPDU,the MSTP entity 408 retrieves the MST Configuration ID from field 138(FIG. 1) and provides it to the comparator 423. Comparator 423 comparesthe MST Configuration ID from the BPDU with switch's own MSTConfiguration ID stored at STP memory 424. If all four values match,i.e., they have the same Island names, the same Configuration names, thesame Revision Levels and the same Configuration Digests, then thereceived BPDU is utilized by the switch in its computation of activetopologies. That is, the BPDU is presumed to have been sent by anotherswitch in the same Island or by the same switch but from a differentInter-Island port.

[0106] Where an Island has multiple connections to an Inter-IslandTrunk, this ensures that, for each PE-VLAN ID defined within a givenIsland, there is only one port connecting the Island to the Inter-IslandTrunk. In other words, the port role selection state machine 414 and theport transition state machine 416 transition only one such port to aforwarding state. All other ports are transitioned to the blocking statefor this PE-VLAN ID. In addition, each VEC is mapped to a single PE-VLANID. Thus, frames associated with a given VEC ID can only be sent andreceived from a single port coupled to the respective Inter-IslandTrunk. Also, the MAN Provider configures the Islands so that the onlyconnections between the Islands are Inter-Island Trunks, and that anygiven VEC is carried on no more than one Inter-Island Trunk. Thecombination of these steps, prevents the formation of loops.

[0107] If an Island has exactly two Inter-Island Ports onto a givenInter-Island Trunk, they are preferably configured as point-to-pointlinks so as to take advantage of the rapid spanning tree convergenceproperties of MSTP/RSTP.

[0108] If the Island ID field 502 of the MST Configuration ID 500matches that stored by the receiving switch, but any other part of theBPDU's MST Configuration ID does not match, i.e., the ConfigurationName, the Revision Level and/or Configuration Digest value aredifferent, then the switch treats the received BPDU as a conventionalRSTP. That is, the switch utilizes the information in the BPDU's outerpart 104 (FIG. 1) to cooperate in the calculation of a single CIST withthe bridge that sourced the BPDU, but ignores the information in theinner part 106. This situation might occur when the MAN Provider is inthe process of updating the VID/MSTI Translation Tables, and thusRevision Levels, of the switches located within a given Island.

[0109] If the Island ID 502 specified in the received BPDU does notmatch the Island ID stored by the switch in its STP memory 424 and theRoot ID identified in the BPDU's CIST Root ID field 116 does not matchthe corresponding value stored at the STP memory 424, then the MSTPentity 408 ignores and discards the received BPDU. In this case, thereceived BPDU is presumed to have been sent by a switch disposed in someother Island. It is a precondition to connecting a bridge to the BPDUVEC to ensure that the choice of Island names is consistent with thenames used by other switches connecting to the same BPDU VEC. The MANProvider may accomplish this through administrative action, e.g., bycorrectly setting the contents of the STP memories 424 of the respectiveswitches.

[0110] If the Island ID specified in the received BPDU does not matchthe switch's Island ID, but the Root ID in field 116 does match, theMSTP entity 408 preferably transitions the port on which the BPDU wasreceived to the blocking state for all VLANs and issues an alarm to theMAN Provider. This situation reflects a mis-configuration of the MANProvider's Islands. Specifically, it suggests that two different Islandsare interconnected by a link(s) other than an Inter-Island Trunk.Furthermore, if at any port other than an Inter-Island Port, a BPDU isreceived whose Island name does not match the receiving switch's Islandname or which is not an MST BPDU, then the port is blocked for allPE-VLAN IDs, and an operator alarm is signaled.

[0111] Similarly, if a BPDU is received that does not have an Island IDfield 502, it is 5 discarded and not relied upon by the receiving bridgein its spanning tree calculations.

[0112]FIG. 9 is a highly schematic, partial block diagram of network 200illustrating Inter-Island Trunk 242 disposed within Island InterconnectFabric 208 and configured to carry traffic for the second VEC. Asdescribed above, the second VEC extends between Islands 202 and 204.Each of these Islands 202 and 204, moreover, have two Inter-Island links210 a, 210 b and 210 c and 210 d, respectively. Each Island 202 and 204prevents the formation of a loop that would otherwise be caused by theexistence of Inter-Island Trunk 242 by placing all but one of its portscoupled to Inter-Island Trunk 242 in the blocking state. For example,the port coupled to Island link 210 b at Island 202 and the port coupledto Island link 210 d at Island 204 may each be transitioned to theblocking state, as indicated by dots 902 and 904. The portscorresponding to links 210 a and 210 c, on the other hand, are eachtransitioned to forwarding.

[0113]FIG. 10 is a highly schematic, partial block diagram of MAN 200illustrating Inter-Island Trunk 244 disposed within Island InterconnectFabric 208 and configured to carry traffic for the third VEC configuredto connect customer sites 216 and 217 (FIG. 2) via Islands 204 and 206.Islands 204 and 206 are coupled to Inter-Island Trunk 244 viaInter-Island links 210 c, 210 d and 210 e and 210 f. Each Island 204 and206 prevents the formation of a loop that would otherwise be caused bythe existence of Inter-Island Trunk 244 by placing all but one of itsports coupled to Inter-Island Trunk 244 in the blocking state. Forexample, the port coupled to Island link 210 c at Island 204 the portcoupled to Island link 210 e at Island 206 may each be transitioned tothe blocking state, as indicated by dots 1002 and 1004. The portscorresponding to links 210 d and 210 f transition to forwarding.

[0114] Suppose that Islands 202, 204 and 204 are further configured toprovide a fourth VEC for interconnecting customer sites 213, 216 and217. FIG. 11 is a highly schematic, partial block diagram of MAN 200illustrating an Inter-Island Trunk 1100 disposed within IslandInterconnect Fabric 208 that has been configured to carry traffic forthe fourth VEC. Here, all four Inter-Island links 210 a-d connect toInter-Island Trunk 1100. To prevent the formation of loops, each Island202, 204 and 206 places all but one of its ports coupled to Inter-IslandTrunk 1100 in the blocking state. For example, the port coupled toIsland link 210 c at Island 204, the port coupled to Island link 210 dat Island 204 and the port coupled to Island link 210 f at Island 206may each be transitioned to the blocking state, as indicated by dots1102, 1104 and 1106. The ports corresponding to links 210 b, 210 c and210 e each transition to forwarding.

[0115] As shown in FIGS. 9-11, although each Island is coupled to theIsland Interconnect Fabric by multiple Inter-Island links, the formationof loops are specifically avoided. In addition, traffic is load-sharedamong the Inter-Island links 210.

[0116]FIG. 12 is a highly schematic illustration of another Inter-IslandTrunk 1200 in accordance with the present invention. The IslandInterconnect Fabric has been omitted for clarity. Inter-Island Trunk1200 includes a plurality of Islands 1202-1210. Each Island, moreover,has a plurality of interconnected bridges. As shown, there are threeVECs 1212-1216 formed among the Islands 1202-1210, all carried on asingle Inter-Island Trunk. Island 1202 has only a single connection 1215to VEC 1212. Therefore, if connection 1215 is lost, Island 1202 losesconnectivity with Islands 1204 and 1210. The bridges of Island 1208 areorganized into two parts, part 1218 a and 1218 b, each made up of fourinterconnected bridges. However, there are no connections between thebridges forming the two parts 1218 a and 1218 b inside of Island 1208.Instead, the two parts 1218 a and 1218 b of Island 1208 utilize VECs1214 and 1216 for intercommunication. Similarly, at Island 1206,execution of the IMCP results in link 1219 between the two bridges beingblocked. The two bridges of Island 1206 utilize VEC 1216 tointercommunicate.

[0117]FIG. 13 is a highly schematic illustration of the sameInter-Island Trunk 1200 as FIG. 12. However, the VECs have been omittedfor clarity and the BPDU VEC or BPDU Service Instance 1302 is shown. Asmentioned above, each Inter-Island Bridge of the Islands 1202-1210 thatare connected to an Inter-Island Trunk have a connection to the BPDU VEC1302. As described herein, the Inter-Island Bridges utilize the BPDU VEC1302 to exchange BPDUs among themselves. The Inter-Island Bridges usethese received BPDUs in their execution of the IMCP to identify andblock redundant links to the VECs. For those Inter-Island Bridgesconnected to multiple VECs on a given Inter-Island Trunk, only a singleconnection is required to the BPDU VEC 1302. For example, bridge 1304 atIsland 1210 which is connected to VECs 1212 and 1214 (FIG. 12) need onlyestablish a single connection 1306 to the BPDU VEC 1302.

[0118] Within each Island, the Island Boundary Bridges run the IMCP ontheir Inter-Island Ports. For the other ports, i.e., non Inter-IslandPorts, within an Island, either the IMCP or MSTP may be run.

[0119] In an alternative embodiment, the unmodified IEEE P802.1S/D13Multiple Spanning Tree protocol may be used instead of the IMCP. In thiscase, each Inter-Island Trunk must have a separate BPDU VEC for eachIsland. Conversely, each Inter-Island Bridge must be configured, foreach Inter-Island Trunk, to attach to the same BPDU VEC as the otherInter-Island Bridges in that same Island. Furthermore, if multipleInter-Island Trunks are employed, then the set of Islands interconnectedby each Inter-Island Trunk's BPDU VECs must be identical. Thus, theconnectivity of the BPDU VECs defines the Islands, rather than thecomparison of Island IDs. Since the Inter-Island Bridges of differentIslands are not interconnected on any BPDU VEC, they cannot detect andreport erroneous connections between Islands that do not utilizeInter-Island Trunks.

[0120] As mentioned above, there are different categories of VECs. TheVECs described above correspond to “bridge-like” VECs in which theCE-VLAN IDs of received frames are altered within the Island.Additionally, network messages corresponding to L2 protocols that arenot used for customer-MAN interaction, such as IEEE Std. 802.3-2000pause frames (also known as 802.3x pause frames) are discarded uponreceipt at the UNI. As indicated above, BPDUs from the customer sitesare never utilized by the switches disposed in the Islands in theircomputation of the CIST. With “wire-like” VECs, CE-VLAN ID tagged framesare carried transparently through the MAN as are network messagescorresponding to L2 protocols that are not used for customer-MANinteraction.

[0121] It should be further understood that an Island may consist of asingle L2 switch. In this case, the PE-VLAN IDs are confined to thesingle switch.

[0122] The foregoing description has been directed to specificembodiments of this invention. It will be apparent, however, that othervariations and modifications may be made to the described embodiments,with the attainment of some or all of their advantages. Therefore, it isan object of the appended claims to cover all such variations andmodifications as come within the true spirit and scope of the invention.

What is claimed is:
 1. A method of preventing Layer 2 (L2) loops in aMetropolitan Area Network (MAN) having a plurality of intermediatenetwork devices, and providing a plurality of Virtual EthernetConnections (VECs), each representing a virtual shared medium, themethod comprising the steps of: organizing the plurality of intermediatenetwork devices into two or more administrative groups, each containingone or more intermediate network devices; using an Interconnect Fabricto couple the two or more administrative groups by providing redundantlinks between each administrative group and the Interconnect Fabric; andfor each VEC provided by an administrative group, blocking all but oneof the redundant links to the Interconnect Fabric.
 2. The method ofclaim 1 further comprising the steps of: defining one or more logicalTrunks within the Interconnect Fabric, each logical Trunk representing ashared medium connecting two or more administrative groups; assigningeach VEC to no more than one logical Trunk; within each administrativegroup, defining a plurality of Provider Equipment Virtual Local AreaNetwork (VLAN) Identifiers (IDs); and within each administrative group,associating a given VEC with exactly one PE-VLAN ID.
 3. The method ofclaim 2 wherein the PE-VLAN IDs defined within two administrative groupsand associated with the same VEC are dissimilar.
 4. The method of claim1 wherein the step of blocking comprises the steps of: providing aseparate VEC for configuration bridge protocol data unit (BPDU) messagesgenerated by the intermediate network devices of the administrativegroups of the MAN; and assigning a Multiple Spanning Tree (MST)Configuration Identifier (ID) to each intermediate network device of theMAN, the MST Configuration ID specifying an Island name.
 5. The methodof claim 4 further comprising the steps of: receiving from a logicalTrunk one or more BPDU messages associated with the VEC at a givenintermediate network device of an administrative group, each receivedBPDU message specifying an MST Configuration ID and a first root; andusing the received BPDU message in computing a spanning tree instance inaccordance with the Multiple Spanning Tree Protocol (MSTP), providedthat the MST Configuration ID of the received BPDU matches the MSTConfiguration ID assigned to the given intermediate network device. 6.The method of claim 5 wherein the MST Configuration IDs further specifya Configuration name, a Revision level and a Configuration digest, andtwo MST Configuration IDs match where the specified Island names,Configuration names, Revision levels and Configuration digests allmatch.
 7. The method of claim 6 further comprising the steps of: storinga root ID at the given intermediate network device; and discarding thereceived BPDU, if the Island name of the received BPDU's MSTConfiguration ID Island ID does not match the Island name of the MSTConfiguration ID assigned to the given intermediate network device, andthe received BPDU's first root does not match the root ID stored by thegiven intermediate network device.
 8. The method of claim 4 wherein theMST Configuration IDs further specify a Configuration name, a Revisionlevel and a Configuration digest, and the step of blocking furthercomprises the steps of using the received BPDU in computing a spanningtree instance in accordance with the Rapid Spanning Tree Protocol (MSTP)specification standard, if the Island name of the received BPDU matchesthe Island name of the given intermediate network device, but one ormore of the Configuration name, Revision level and Configuration digestof the received BPDU does not match the respective one of theConfiguration name, Revision level and Configuration digest assigned tothe given intermediate network device.
 9. The method of claim 1 whereineach administrative group of the MAN is identified as a correspondingIsland, and a plurality of customer networks are coupled to each Island.10. The method of claim 9 wherein the customer networks sendconfiguration bridge protocol data unit (BPDU) messages into theirrespective Islands, the method further comprising the step of returningBPDU messages generated in the customer networks back to the customernetworks unmodified.
 11. The method of claim 1 wherein the step ofblocking comprises the steps of: configuring a logical Trunk so thateach intermediate network device can communicate configuration bridgeprotocol data unit (BPDU) messages over the logical Trunk only withintermediate network devices that belong to the same administrativegroup; and using the received BPDU messages in computing a spanning treeinstance in accordance with the Multiple Spanning Tree Protocol (MSTP).12. An intermediate network device for use in forwarding networkmessages within a computer network, the intermediate network devicecomprising: a plurality of ports configured to send and receive thenetwork messages; means for associating the intermediate network devicewith an Island name, a Configuration name, a Revision level and aConfiguration digest; means for issuing configuration bridge protocoldata unit (BPDU) messages with the Island name, Configuration name,Revision level and Configuration digest associated with the intermediatenetwork device; and means for utilizing one or more received BPDUmessages in computing a spanning tree instance provided that each of theIsland name, Configuration name, Revision level and Configuration digestof the one or more received BPDU matches the respective Configurationname, Revision level and Configuration digest associated with theintermediate network device.
 13. The intermediate network device ofclaim 12 further comprising means for preventing one or more receivedBPDU messages from being used in computing a spanning tree instancewhere the Island name of the received BPDU does not match the Islandname associated with the intermediate network device.